Inductive coupling arrangement



1946. I E. o. WILLOUGHBY 2,399,779

INDUCTIVE COUPLING ARRANGEMENT Filed April 24, 1942 L62 LV LCI 9 MWFA/TOA Patented May 7, 1946 INDUCTIVE COUPLING ARRANGEMENT Eric Osborne Willoughby, London, England, assignor to International Standard Electric Cor- .Poration, New York, N. Y.

Application April at, 1942, Serial No. 440,292

r In Great Britain May 27, 1941 Claims. (Cl. 178-44) The present invention relates to inductive coupling arrangements betweena tunable feed circuit having a' variable inductance ganged to the tuning element of a related tunable circuit and a-condenser of constant capacity and a tunable load circuit having a coupling inductance coupled to the inductance of the feed circuit.

When the setting of the tuning members of the circuits is varied to change the operation from one operating frequency to another, it happens that the coupling between the feed circuit and the load circuit'also varies and the power delivered to the load varies with the operating frequency so that in order to maintainthe power delivered to the load constant for all frequencies it is necessary to adjust the coupling for each particular frequency. This separate adjustment is not always convenient as for instance on radio transmitters on aircraft where the output of an amplifier circuit feeds into an antenna which constitutes the load and to change from one operating frequency to another, it is desired that a minimum number of operations should be performed. It is-'an object of this invention to eliminate the necessity for adjusting the coupling between the feed and load circuits for each operating frequency and to Provide an arrangement in which the coupling between the two said circuits is automatically adjusted for all wave lengths in the operating range.

According to one aspect of the invention an inductive coupling arrangement of the type hereinbefore specified is characterised in this that the variable inductance in the feed circuit comprises two helical coils connected in series and coaxially disposed and one of which at least has a variable number of turns adjusted so that the feed circuit is purely resistive at the operating frequency and the said coupling inductance comprises one or more coupling coils connected in series, one or more of the factors on which the mutual inductance of the two coupled coils depends, viz. size, shape, disposition and number of turns of the coupling coil or coils and the disposition of said coupling coil or coils with respect to the said variable inductance being so chosen that for variations in the'said variable induct-' ance for diiferent frequencies the mutual inductance between the feed and load circuits is substantially at the appropriate value for constant power in the load circuit throughout the operating frequency range.

According to another aspectof the invention an inductance coupling arrangement of the type hereinbefore specified is characterised in this that the variable inductance in the feed circuit comprises two helical coils connected in series and coaxially disposed and one at least of which has a variable number of turns and the coupling inductance comprises one or more coupling coils connected in series, the said coupling coil or one or more of the coupling coils being over-coupled as determined by the load resistance to the part of the said variable inductance effective to tune the feed circuit at the shorter wave end of the operating wave range, this over-coupling in the feed circuit being compensated by adjustment of the constant capacity of saidfeed circuit so that on tuning the load circuitfor maximum power the coupling is at the appropriate value for constant power output for all frequencies within the operating range.

j The invention will be further described in relation to an embodiment thereof in which an amplifier circuit feeds into a radio transmitter antenna circuit, the tuning coil of the output circult of the amplifier and the tuning coil or coils of preceding frequency controlling circuits being anged. Such an embodiment is shown diagrammatically in Fig. 1 of the accompanying drawing,

whilst Fig. 2 shows an alternative arrangement of the coupling coils.

The most important field of application of the invention is to aircraft transmitter antennae, operating at wavelengths such that the antenna circuit electrical length is less than a quarter wavelength throughout the working frequency range of the transmitter.-

In such antenna circuits two adjustments are necessary, for any variation in tuning of the last stage amplifier output or tank circuit, this latter being carried outautomatically 0n adjusting the master oscillator circuit on a ganged transmitter. The two adjustments referred to are tuning the antenna circuit to resonance at the operating frequency and a correcting adjustment of the coupling between the antenna circuit and output valve tank circuit.

The object of the present invention is to ellmimate the coupling adjustment. Referring to Fig. l the output circuit of the last stage amplifier V comprises a tunable circuit consisting of and of the operating range, whilst the coil LF is assumed to have a constant number of turns and is approximately the correct tuning inductance at the shorter wave end of the operating range. LF also has a high "Q factor, 1. e. ratio of inductive impedance to resistance.

The antenna circuit AE comprises a variable inductance LA for tuning the antenna circuit, and a coupling coil LC of a constant number of turns disposed between the two coaxial helical coils LF, LV so as to be inductively coupled thereto.

Two cases require consideration, the first be-' ing the case in which the output tank circuit is tuned to be purely resistive at each operating frequency:

In this adjustment of the arrangement the active portion X of the output stage tuning coil, as determined by the position of the adjustable member R for varying the number of turns of the inductance coil in circuit, automatically varies the mutual inductance between the output tuning inductance coil LV, LF and the coupling coil LC, and one or more of the factors controlling the mutual inductance, namely the size, number of turns, shape and position of the coupling coil LC with respect to the tuning coil LV, LF is/are so chosen that the mutual inductance between the tuning and coupling coil varies with change of position of the adjustable tuning member so that the mutual inductance is automatically maintained substantially at the correct value to give constant power output throughout the operating frequency range.

With ganged tuning inductances covering a frequency range, the operation of the transmitter at the shorter wave end of its range is considerably improved by dividing each tuning coil into two units, a coil LF of constant inductance equal to that necessary to tune the fixed capacity at the shorter wave end of the operating range, and a variable coil LV over which the adjustable tuning contact R moves to cover the frequency variation. The Q (i. e. ratio of inductive impedance to resistance) of the tuning coil is thereby maintained at a high value at the shorter wave end of the operating range, and effects due to high tank circuit losses at the shorter wave end of the operating range are minimised.

This'type of construction lends itself readily to automatic coupling adjustment as it is relatively easy to place a coupling coil LC between the constant coil LF and vaiiable coil LV of the tuning inductance and the coupling is adjusted to the correctvalue at the highest and lowest irequencies, for instance, by adjusting the position and number of turns on the coupling coil LC.

By such expedients the radio' frequency current through a fixed load resistance was maintained constant over a 22:1 frequency range to "within when both output tank circuit and antenna circuit were tuned to be purely resistive.

If two coupling coils LCI and L02 in series are used, one LCI coupled to the constant inductance coil LF and the other, LC2 to the variable inductance coil LV as shown in Fig. 2, even better results can be obtained, but the preferred adjustment is that described hereinafter, this adjustment being more convenient from a manufacturing point of view and giving a more uniform coupling law. The amplifier output tank circuit and the antenna circuit are not tuned to be resistive in this case, but are tuned to have a much lower power factor at the shorter wave end of the operating range than at the'longer wave end.

The arrangement thus adjusted-is applicable to ganged transmitters where the output coil moves in a predetermined manner. I

The essentials of the preferred adjustment are as follows:

(a) The output circuit is over coupled at the shorter wave end of the range thereby providing some of the large coupling required at the longer wave end of the operating range.

(b) This over coupling-is compensated by introduced reactance in the amplifier output tank circuit by varying the capacity of the fixed condenser from the value correct for ganging, by a small amount, which is sufiicient to correct the coupling at the shorter wave end of the tuning range when the antenna circuit is tuned for maximum power output. For this purpose a trimmer condenser CT is provided in parallel with themain condenser C.

The power output is then checked at the longer wave end of the tuning range with this adjustment and the trimmer condenser and coupling are manipulated until the conditions are adjusted for the correct power output at the two ends of the tuning range. I

It should be noted that the size of the coupling coil is determined almost entirely by practical considerations and the number of turns is obtained in early approximate adjustments, and once the coil size and turns have been determined, the coupling and tuning condensers are adjusted as explained above.

Since the coupling law is a smooth one, it is only necessary to obtain correct coupling at the extreme ends of the tuning range and to make one coupling check in the middle of the range.

If there is inadequate coupling in the middle of the coupling characteristic, it is advisable to over-couple more at the shorter wave end of the operating range and readjust the trimmer condenser to give the correct power output at the short wave end, again checking the power output at the longer wave end and at the centre of the operating wave range.

By this method extremely good constancy of load coupling over a frequency range of 2.511 can be obtained, without dlflicult adjustment or requirement of special components.

The coupling is adjusted by positioning LC relative to LV and LF and the number of turns on LC, and the condenser C by adjustment of the tuning condenser CT.

In practice it is found most convenient to make up LC of two groups of turns spaced towards the ends of the coil former thus enabling the couplings to the constant and variable coils to be increased for a given coupling 'coil inductance and improving the linearity of the relation between the coupling and operating frequency, or alternatively the coupling coil LC can be made up of two separate coupling coils in series as shown in Fig. 2 if greater control over the said relation is required.

Elementary analysis shows that the conditions for unity power factor (P. F.) operation of the output valve V are satisfied when the 2/1' ratio (series reactance divided by series resistance) in the amplifier output tank circuit and the antenna circuit are equal to each other, or when x/r of one circuit is equal to its reciprocal quantity for the other circuit.

This latter property, however, need cause no difficulty in practice as the coupling-condition obtained by the procedure of tuning the antenna circuit to maximum antenna current, but it gives rise to the apparent anomaly that the coupling unit is also apparently satisfactory for another load resistance R Then RR =X", It being the series resistance of the valve output circuit and X being the antenna reactance necessary for correct tuning. This new tuning condition, however, results in reduced loading over the greater part of the operating range and causes no ambiguity in practice, being only discovered on using a load other than that for which it is designed.

What is claimed is:

1. An inductive coupling arrangement comprising a tunable feed circuit having a condenser of constant capacity when the frequency of the current passing through said arrangement lies between predetermined limits, and an inductance comprising first and second coaxially disposed longitudinally spaced helical coils connected in series and with the first coil having one end-open and thus inactive, means associated with said first coil for varying'the number of turns of said coil included in said feed circuit, and a tunable load circuit having at least one inductance coil coupled to said first-mentioned inductance and positioned adjacent the inactiveend of said first so coil.

2. An inductive coupling arrangement accordtween said first and second coils and coaxial therewith.

3. An inductive coupling arrangement according to claim 1 wherein said tunable load circuit 4. An inductive coupling arrangement compris- 5 ing a tunable feed circuit having a condenser of constant capacity when the frequency of the current passing through the arrangement lies between predetermined limits and first and second coaxially-dispo'sed helical coils, longitudinally displaced with respect to one another, a connection between the inner end of said first coil and the outer end of said second coil, a variable tapping on said second coil, and a coupling coil disposed between and coaxial with said first and second coils.

5. An inductive coupling arrangement comprising a tunable feed circuit having a condenser of constant capacity when the frequency of the cur- I rent passing through the arrangement lies bepositioned adjacent the outer end of a respective tween predetermined limits and first and second coaxially-disposed helical coils, longitudinally displaced with respect to one another, a connection between the inner ends of said coils, a variable tapping on one of said coils, and two series-connected coupling coils each of which is one of said first and second coils and is coaxial therewith.

ERIC OSBORNE WILLOUGHBY. 

